The present invention relates to viscous fluid couplings, and more particularly, to such couplings which are used to drive vehicle radiator cooling fans, wherein the engagement or disengagement of the viscous fluid coupling is controlled in response to a remotely sensed condition, such as coolant temperature.
Although the present invention is not limited to use in a viscous fluid coupling which is controlled "remotely", the use of the invention is especially advantageous in such couplings, and the invention will be described in connection therewith.
Viscous fluid couplings have been in commercial use for many years to drive vehicle radiator cooling fans, especially on vehicles such as automobiles and light trucks. The use of such viscous couplings has been widespread, and quite desirable because such couplings can operate in either an engaged condition, or a disengaged condition, depending upon a sensed temperature condition. Most of the fluid couplings (viscous fan drives) which have been in commercial use include some form of temperature-sensing bimetallic element which senses ambient air temperature adjacent the fan drive, and controls valving within the fan drive in response to the sensed temperature, to achieve either the engaged or the disengaged condition, as is appropriate.
In certain vehicle applications, it has become desirable to sense directly the temperature of the liquid coolant entering the radiator ("top tank" temperature), and to control the viscous fan drive in response to the coolant temperature, whereby the responsiveness of the fan drive is improved, when compared to the conventional fan drive which senses the ambient air temperature.
In order to improve the ability to control such remote-sensing fan drives, it has been considered desirable to convert the sensed temperature into an electrical signal, and to control the engagement of the fan drive by means of that electrical signal. Such electrical control also makes it possible to incorporate more sophisticated logic in the control system. U.S. Pat. No. 5,152,383, assigned to the assignee of the present invention, and incorporated herein by reference, discloses a remote-sensing viscous fan drive in which there is electromagnetic control, by means of variable reluctance, of a rotatable armature, relative to a stationary housing, thereby controlling the rotary position of a valve element in the fan drive, to either cover or uncover a fluid inlet port.
In the device of the referenced patent, the electromagnetic control includes a tethered housing through which extends an armature shaft, connected to the valve element of the fan drive. The valve element illustrated therein is of fairly conventional construction. Among the trade-offs involved in the design of such a fan drive is the need, on the one hand, to have a good seal between the valve and the adjacent surface defining the fill port (to minimize "morning sickness"), and the need, on the other hand, to be able to rotate the valve element without requiring an excessively large and expensive electromagnetic actuator. As is well known to those skilled in the art, those two objectives are generally contradictory, e.g., increasing the sealing force of the valve element results in the need for more torque to rotate the valve element. Conventionally, improving the sealing of the valve member has involved tighter tolerances on the various components, which adds to the overall cost of the coupling device, an approach which is undesirable. Also, improving the sealing can have a negative impact on response time, i.e., the time required for the coupling device to move from an engaged condition to a disengaged condition, or vice versa.